In a food
processor, chop the nuts coarsely, then add the ginger, salt, tomato
slices, chilies, shallots, scallions, and red onion. Whirl to make a
paste, then stir in the peanut oil.

Place
one-third of this mixture in the middle of the aluminum square, place fish
on top of the paste, and place another one-third of the paste in the
cavity of the fish. Scrape remaining paste over fish and fold to make a
secure package.

Place package
over boiling water in a large steamer and steam, covered, for 30 minutes.
Replenish water from time to time, if necessary.